That being said, your point is still valid. Just in this case the heavy data
is accessed "rarely".
- ryan.
On Wed, Oct 14, 2009 at 9:27 AM, Andy Freeman <ana...@earthlink.net> wrote:
>
> > > So now I am thinking of breaking up the WriteUps class into several
> > > classes, putting all the lightweight data that I need to access
> > > frequently in one class and all the heavy data that I rarely need to
> > > access in another and using ancestor relationships and transactions to
> > > make sure everything stays together.
> > > Does this make any sense, or I am fundamentally misunderstanding how
> > > app engine fetches data?
> >
> > Yes, that sounds like a good idea.
>
> Where's the tradeoff point?
>
> For example, suppose that the heavy data is accessed 90% of the time,
> but all of the accesses are via a single db.get and the heavy data
> instance is always in the same entity group. ("via a single db.get"
> means that if the heavy data is in a separate instance and it's
> needed, the same db.get is used for both the light data and the heavy
> data instances).
>
>
> On Oct 13, 3:49 am, "Nick Johnson (Google)" <nick.john...@google.com>
> wrote:
> > Hi Shailen,
> >
> >
> >
> >
> >
> > On Sun, Oct 11, 2009 at 4:09 PM, Shailen <shailen.t...@gmail.com> wrote:
> >
> > > Consider 2 classes:
> >
> > > class Light(db.Model):
> > > location = db.StringProperty()
> >
> > > class Heavy(db.Model):
> > > location = db.StringProperty()
> > > content = db.TextProperty()
> > > image1 = db.BlobProperty()
> > > image2 = db.BLobProperty()
> > > image3 = db.BlobProperty()
> > > ..., etc., etc. ...
> >
> > > I have 2 choices when getting information from the datastore: I can
> > > either fetch just the keys, or I can
> > > fetch the entities themselves. There isn't any way to fetch only a
> > > part of an entity (say just the location property in entities of type
> > > Heavy). Assuming this is correct, I have a couple of questions:
> >
> > > 1) If I want to access the location property in both Light and Heavy,
> > > is it fair to assume that accessing Light will be speedier than
> > > accessing Heavy, since Heavy has to fetch all sorts of data that I
> > > don't need but that is part of the entity?
> >
> > Yes.
> >
> >
> >
> > > 2) Would it make sense to break down Heavy into more lightweight
> > > classes for more efficient lookup?
> >
> > In the example above, definitely, yes.
> >
> >
> >
> > > This question is probably best clarified by discussing what's
> > > happening in my actual application where I allow an Author to create
> > > several WriteUps. The WriteUps have numerous properties: a heading, a
> > > sub-heading, a lot of text and, optionally, several images, and a few
> > > more properties. When displaying the contents of a WriteUp, I need to
> > > access all these properties. But sometimes, I need to access only 1 or
> > > 2 lightweight properties; for instance, on a page summarizing the
> > > WriteUps done by an author, I need to only list the heading and sub-
> > > heading; I do not need to access the text or Blob data. But there is
> > > no way to access *just* the heading or sub-heading, right? I end up
> > > fetching ALL of the entity, whether I need all of it or not. Is this
> > > correct? Or does the datastore have some way of optimizing this?
> >
> > Entities are always fetched in their entirety.
> >
> >
> >
> >
> >
> >
> >
> > > Reading the In the documentation (in the Queries and Indexes section),
> > > we are told:
> >
> > > # The query is not executed until results are accessed.
> > > results = q.fetch(5)
> > > for p in results:
> > > print "%s %s, %d inches tall" % (p.first_name, p.last_name,
> > > p.height)
> >
> > > When are results 'accessed', when fetch() is called, or when
> > > p.first_name, p._last_name, etc. are used? I am assuming that when
> > > fetch(5) is called, all 5 entities (assuming they exist) are fetched,
> > > and if the entities contain properties other than first_name,
> > > last_name or height, they are loaded up too. Is that correct?
> >
> > Yes.
> >
> >
> >
> > > So now I am thinking of breaking up the WriteUps class into several
> > > classes, putting all the lightweight data that I need to access
> > > frequently in one class and all the heavy data that I rarely need to
> > > access in another and using ancestor relationships and transactions to
> > > make sure everything stays together.
> > > Does this make any sense, or I am fundamentally misunderstanding how
> > > app engine fetches data?
> >
> > Yes, that sounds like a good idea.
> >
> > -Nick Johnson
> >
> >
> >
> > > - Shailen
> >
> > --
> > Nick Johnson, Developer Programs Engineer, App Engine
> > Google Ireland Ltd. :: Registered in Dublin, Ireland, Registration
> Number:
> > 368047- Hide quoted text -
> >
> > - Show quoted text -- Hide quoted text -
> >
> > - Show quoted text -
> >
>
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